A. Field of the Invention
The present invention relates generally to the field of altering currents in the presence of an applied electric field or applied current source within biological material and more particularly to a method and apparatus to generate displacement currents in living tissue by altering local tissue permittivity characteristics via mechanical, electrical, optical, chemical, and/or thermal means relative to an applied electric field to stimulate biological tissue.
B. Background Information
Electric stimulation of living tissue in humans and other animals is used in a number of clinical applications as well as in clinical and general biological research. In particular, electric stimulation of neural tissue has been used in the treatment of various diseases including Parkinson's disease, depression, and intractable pain. Focused stimulation of the brain usually involves performing surgery to remove a portion of the skull and implanting electrodes in a specific location within the brain tissue. The invasive nature of these procedures makes them difficult and costly, and is responsible for a great deal of morbidity. Alternately, noninvasive stimulation methodologies such as transcranial direct current stimulation and transcranial magnetic stimulation are easy to implement and are not associated with significant morbidity, however, the areas stimulated are large, typically not well characterized, and can be significantly perturbed by natural or pathological features of the brain tissue. Recently, ultrasound stimulation of brain tissue has been explored with limited success.
Numerous methods exist for generating currents for biological tissue stimulation. These methods range from implanting electric sources in the tissue to inductively generating currents in tissue via time-varying magnetic fields. A common method for generating currents in tissues is to implant current sources within the tissue. Examples of this method are illustrated, for example in U.S. Pat. No. 5,895,416 to Barreras, Sr. et al., U.S. Pat. No. 6,128,537 to Rise, U.S. Pat. No. 7,146,210 to Palti, and U.S. Pat. No. 6,091,992 to Bourgeois et al. Currents can also be produced in tissues with sources external to the tissues, such as via external magnetic fields which induce currents in tissues. This method is shown, for example, in U.S. Pat. No. 6,066,084 to Edrich et al., U.S. Pat. No. 5,061,234 to Chaney, U.S. Pat. No. 6,234,953 to Thomas et al. Another example is shown in U.S. Pat. No. 7,146,210 to Palti which implements electromagnetic radiation. Methods employing currents produced via electric sources placed in external contact to the tissue such that the currents attenuate through other tissues superficial to the region of tissue to be stimulated are illustrated in U.S. Pat. No. 4,989,605 to Rossen and U.S. Pat. No. 4,709,700 to Hyrman. None of these techniques generate currents via a permittivity perturbation in the presence of an applied electric field. As such, these techniques suffer from limitations in the level of invasiveness, focality, penetration, and/or cost.
The concept of combining fields for the generation of altered displacement currents is relatively unexplored in the area of biological tissue stimulation. In the area of brain stimulation, magnetic fields have been explored with ultrasound techniques in the area of “hall effect stimulation,” for example, as in U.S. Pat. No. 5,476,438 Edrich et al., whereby “ionized particles within the nerve tissue and, particularly, electrons are mobilized” such that positive and negative ions are separated in the area of the orthogonal magnetic field where the ions are moving under the influence of ultrasound. This method does not attempt to generate a displacement current through the modification of tissue permitivitties, but rather just local ionic separation via applying a magnetic field to moving ions. With the strength of magnetic fields used in modern medical procedures, this technique is ineffective for stimulation. See Rutten, et al. (1996). Also in the area of brain stimulation, U.S. Pat. No. 6,520,903 to Yamashiro proposes a method to enhance energy transfer of magnetic fields by photonic fields focused on the tissue, but it does not attempt to generate a displacement current via a tissue permittivity perturbation. Additionally, in the area of brain stimulation, U.S. Pat. No. 5,738,625 to Gluck (hereinafter “Gluck”) proposes the use of magnetic fields with a combined ultrasound field and/or microwave fields in order to change the membrane potential of a neuron to a static value significantly different from the cell's resting potential and a separate active depolarized state. Gluck proposes the modification of tissue conductivity via ultrasound such that currents induced by a magnetic field could flow on the paths of altered conductivity. Gluck also proposes the use of ultrasound to push nerves in and out of the fields generated by the magnetic field. Gluck implements a method altering which nerves are exposed to a magnetic field (or currents) and thus the magnetic based method of Gluck suffers from a loss in efficiency due to subsequent current attenuation.
Furthermore, Gluck proposes a method in which microwave and ultrasound fields are combined in a way that may lead to non reversible changes to nerves. See Donald I. McRee, Howard Wachtel, Pulse Microwave Effects on Nerve Vitality, Radiation Research, Vol. 91, No. 1 (July, 1982). The present disclosure does not suffer from these safety concerns or cause nerve damage by requiring the use of such high frequency electric fields. In addition, the disclosed invention herein is not constrained to apply only to neural tissue exhibiting distinct states of quiescence and activity, and would therefore be appropriate for dynamically changing action potentials that characterize almost all neural activity and for neurons with dynamic firing properties.
Other methods have been proposed for altering tissue conductivities for adapting current flow, such as U.S. Pat. No. 6,764,498 to Mische and U.S. Pat. No. 6,887,239 to Elstrom et al., but similarly, these methods do not provide a method that generates a new current component through the modification of the tissue electromagnetic properties.
Other studies have proposed techniques to affect neural stimulation with combined fields but all suffer from inherent limitations in that the techniques do not attempt to generate displacement currents for stimulation but attempt to affect stimulation through other means. See Rutten, W. L. C., E. Droog et al.; The influence of ultrasound and ultrasonic focusing on magnetic and electric peripheral nerve stimulation, J. Nilsson, M. Panizza and F. Grandori; Pavia Advances in Magnetic Stimulation, Mathematical Modeling and Clinical Applications, Italy. 2: 152. (1996) (hereinafter “Rutten”); Mihran, R. T., F. S. Barnes et al., Temporally-Specific Modification of Myelinated Axon Excitability in Vitro Following a Single Ultrasound Pulse. Ultrasound Med Biol 16(3): 297-309. (1990) (hereinafter “Mihran”); and Fry, W. J., Electrical Stimulation of Brain Localized Without Probes—Theoretical Analysis of a Proposed Method, J Acoust Soc Am 44(4): 919-31. (1968) (hereinafter “Fry”).
Mihran and Rutten focus on altering ionic stretch receptors in neural elements. Thus by not focusing on the generation of displacement currents through the appropriate combination of electric and mechanical fields, these studies are limited in applicability and effectiveness. More specifically, the Mihran study combines ultrasound with electrical stimulation to test the effects of stretch receptors on nerves. Mihran does not attempt to generate new currents for stimulation. Mihran purposely decouples the electric and mechanical fields. The primary focus of Rutten is to combine ultrasound and transcranial magnetic stimulation (TMS), however, the study attempts electrical stimulation and ultrasound in an attempt to analyze the effects of stretch receptors similar to Mihran. Rutten does not attempt to combine the effects for the generation of new current components or alter the applied stimulatory currents in any way.
Fry presents an idea regarding how to generate a current modification in the brain by modifying the tissue conductivity via ultrasound and thus driving neural stimulation through a conductivity change alone. Fry proposes a theoretical, pseudo invasive, method based on the use of ultrasound and electrodes placed on the brain surface. The method is based on the alteration of tissue conductivity via temperature/pressure changes generated from ultrasound to alter currents generated by the brain surface electrodes. The method has never been shown to work for neural stimulation, possibly because the theory is limited by many constraints. By focusing on modifying just the tissue conductivity to alter currents generated with higher frequency electric fields, the source strengths required for stimulation are not trivial. Therefore, the method necessitates electrodes that must be placed on the exposed brain surface, or much stronger current sources, which if placed on the scalp surface, would suffer from the limitations of Transcranial Eletrostimulation (TES) and Electroconvulsive Therapy (ECT), i.e., current strengths which activate pain receptors on the scalp surface or with strengths necessary for stimulation that may potentially lead to scalp burns. And the ultrasound intensities that are necessary for this theoretical stimulation are large enough in magnitude that concerns arise including temperature rise in the tissue, tissue cavitation, and the possibility of tissue ablation. Thus, these safety limitations would preclude one from applying this type of stimulation for any duration of time, either with electrodes on the surface of the scalp or the brain.
The concept of mechanical and electric fields being interrelated in biological tissues has been explored in the pursuit of imaging as illustrated in U.S. Pat. No. 6,645,144 to Wen, et al. and U.S. Pat. No. 6,520,911 to Wen via electroacoustic, thermoacoustic, and Hall effects. These methods are focused on using one physical field to glean information about the other and not in a combinatory way for biological tissue stimulation.
The prior art techniques do not attempt to generate capacitive currents, i.e., displacement currents, via a permittivity perturbation relative to an applied electric field for biological tissue stimulation. It is thus evident from the above that there is a need for an improved apparatus and method to generate displacement currents in living tissue by altering local tissue permittivity characteristics via mechanical means relative to an applied electric field to stimulate biological tissue. It is evident that there is a need for an improved method for stimulating biological tissue by altering local tissue permittivity that is less invasive and has improved focality. It is further evident that there is a need for an apparatus and method whereby actual currents are generated as opposed to methods where the currents are altered in path or methods altering which nerves are exposed to a magnetic field. It is also evident that there is a need to generate currents below tissue boundaries without subsequent current attenuation and loss in efficiency as takes place with magnetic and electrical based methods. It is evident that there is a need for a safe method that does not cause nerve or tissue damage by requiring the use of high frequency electromagnetic fields, high intensity electromagnetic fields, and/or high intensity ultrasound fields. It is also evident that there is a need for a tolerable method that does not require field strengths that activate pain receptors during stimulation. Additionally, it is evident that there is a need for an apparatus and method that is not constrained to apply only to neural tissue exhibiting distinct states of quiescence and activity, and would therefore be appropriate for dynamically changing action potentials that characterize almost all neural activity and for neurons with dynamic firing properties.